This project is a comprehensive look into the CD4 T cell response to human herpesvirus 6 (HHV-6), an emergent / re-emergent human pathogen. In most people infection in childhood resolves uneventfully to a chronic life-long infection held in check by cellular immune responses. However, viral reactivation after immunosuppression can cause serious illness, neurological complications, and even death, and is an important complication following organ transplantation. One aim of the project is to characterize memory T cell responses to this chronic infection, and compare them to responses to acute infection. CD4 and CDS T cell responses to HHV-6 will be characterized in healthy immune donors and also in kidney transplant recipients experiencing viral reactivation. A possible role for IL-10 in regulating these responses will be investigated. These analysis will involve ex vivo analysis of, peripheral blood and primary cell lines, using functional assays and newly-developed class II MHC oligomers, and will be pursued in collaboration with project 2, which is investigating analogous CD4 T cell functional subsets in mice. A second aim of the project is to understand the role of NK cells in regulating T cell responses to HHV-6. Preliminary data indicate that NK populations present in peripheral blood regulate CD4 T cells responding to HHV-6. Several mechanistic hypotheses will be explored using ex vivo analysis of human peripheral blood and primary cell lines. This work will be pursued in collaboration with project 1, which is investigating similar NK phenomenon in other systems. A third aim of the project is to investigate and functionally characterize CD4 T cell responses to HHV-6 that are cross-reactive with other viruses. Several recently identified HHV-6 T cell epitopes are similar to those from other common viral infections. Based on principles established in previous work on CDS cross-reactivity patterns, we predict that the HHV-6 specific CD4 T cell response can recognize heterologous infection by other viruses including EBV, CMV, and lAV. This prediction will be tested using ex vivo analysis of human peripheral blood and primary cell lines, and biochemical and structural analysis of MHC and TCR proteins. This aim will be pursued in collaboration with project 3.